How to design cold-formed, steel framed, shear wall assemblies

Design provisions for cold-formed steel shear wall assemblies and an understanding of their performance have certainly grown since the early 1990s. Significant advances and additions in the provisions for these systems—such as addition of values for steel and gypsum sheathing, allowance for thicker framing members, shear walls with openings, and deflection equations—have occurred since the inclusion of the first code provisions in the International Conference of Building Officials’ 1997 Uniform Building Code (UBC).

Recently, the American Iron and Steel Institute (AISI) published the 2004 Edition of the Standard for Cold- Formed Steel Framing—Lateral Design. This standard provides additional information and clarifies design and detailing requirements of cold-formed steel lateralforce resisting systems. This article discusses and illustrates the information and requirements of this new standard.

Review of light-framed shear wall behavior

A typical light-framed shear wall transfers lateral loads, in the plane of the wall, through sheathing that is attached with mechanical fasteners to the framing members. The in-plane shear loads are transferred from the shear wall to the floor framing or foundation below along the length of the bottom horizontal member (plate) while the induced overturning forces are transferred through the vertical boundary members (studs) and over-turning restraint system (holddowns) at the ends of the wall as shown in Figure 1.

Typical lateral loads on shear walls result from either wind or seismic demand. Design wind loads are the actual expected wind forces, whereas design seismic loads are reduced from the actual expected seismic forces based on the type of lateral system used, how many lateral elements are used in the structure, and the level of seismic detailing performed. Designing for a reduced seismic load reduces the cost of construction significantly, but the tradeoff is potential damage in the structure during a major earthquake.

Typically, shear wall assembly strengths are determined through monotonic tests per Strength Tests of Panels for Building Construction: Racking Load (ASTM E72) for wind load resistance and cyclic tests per the Sequential Phase Displacement, or the Consortium of Universities for Research in Earthquake Engineering protocol, for seismic resistance.

Member strengths and system failure modes are important considerations for seismic design. Generally, a brittle system is considered one that will fail suddenly, and a ductile system is one that has been designed and detailed to sustain more deformation without loss of load carrying capability. This is done typically by designing the connections and members that are not supposed to yield (or are incapable of yielding, such as compression columns) to have a design strength in excess of that needed to fully develop the strength of the designated yielding elements. Use of the special load combinations that employ the over-strength factor to determine the design level demand of the nonyielding components is one way to ensure that yielding of the designated ductile elements will occur. Codes encourage the use of ductile systems by assigning them a higher R-value, which results in lower required design loads. Advances in design and detailing provisions of cold-formed steel lateral-force resisting systems have increased the possibilities for designers.

Updates in the new standard

Cold-formed steel framed shear wall assemblies have been in the building codes for several years since the initial inclusion in the 1997 UBC. In that code, recognition was given to wood sheathed, 33- and 43-mil cold-formed steel framed shear walls for wind and seismic resistance.

A new AISI standard and commentary includes additional design information and requirements based on the latest research. These additions include gypsum board sheathed assemblies, steel sheathed assemblies, shear wall with openings (Type II), 4:1 aspect ratio allowances in regions of moderate to high seismic risk, shear wall and diaphragm deflection equations, and wood sheathed cold-formed steel framed diaphragm assembly strength.

Shear wall types. The standard recognizes two basic types of cold-formed steel framed shear walls: Type I and Type II. A Type I shear wall is defined as a fully sheathed wall resisting in-plane forces with hold-downs at each end of each wall segment, where "detailing for force transfer around the openings is provided" if the wall has openings. A Type II shear wall is defined as a wall containing multiple wall segments resisting in-plane forces, sheathed in wood or steel that contains openings between wall segments, with hold-downs only at the ends of the wall. There is no requirement to detail for shear transfer around openings in a Type II wall. Figure 2 illustrates Type I and Type II shear walls.

Basically, Type II shear walls use Type I shear wall published strength values modified by a coefficient based on wall and opening height (called the shear resistance adjustment factors). Type II shear walls also have special considerations including design for a uniform uplift force along the wall bottom plates in addition to the typical Type I shear wall design for uniform shear at the bottom plates.

Shear wall tables: The standard has three shear wall tables tabulating nominal strengths based on sheathing material, fastener spacing, framing thickness, and seismic or wind loading. The first table is wood or steel sheathed assemblies resisting wind loads, the second is gypsum board sheathed assemblies resisting wind or seismic loads, and the last table is wood or steel sheathed shear wall assemblies resisting seismic loads.

The values in the tables represent the nominal, or in this case ultimate, wall capacities. These nominal values have to be adjusted to obtain the appropriate design resistance, and this is done by multiplying by a resistance factor (x) to obtain the load and resistance factor design (LRFD) value, or dividing by a safety factor (y) to obtain an allowable stress design (ASD) value. The y is 2.0 for wind and 2.5 for seismic, whereas x is 0.65 for wind and 0.60 for seismic.

A summary of provisions

Some of the standard’s general requirements for shear wall include use of framing members with a minimum thickness of 33 mils, no shear panels less than 12 inches in width, and 24- inch maximum framing spacing. For seismic applications, it is not permitted to have a framing member thickness beyond the limits set in the table. Additionally, summing the strength of shear walls with different sheathing material on the same wall face is not permitted. However, one may increase the wood or steel sheathed shear wall strength by 30 percent if gypsum board is used on the opposite side as permitted by the first table in the standard.

Other specifications are that the wood sheathed and steel sheathed shear panels may be installed either perpendicular or parallel to the framing members and all panel edges are to be blocked. In addition, when the height (h) to width (w) aspect ratio of a shear wall segment is between 2:1 to 4:1, the tabulated nominal shear wall strengths shall be reduced by 2w/h.

Design procedure. A general procedure for design of shear wall assemblies is outlined as follows:

1. Determine design loads (gravity, wind, seismic, etc.).
2. Determine shear wall sheathing/ fastener/spacing/framing type based upon published strengths in the code or standard.
3. Design connection of the member delivering the shear load to the shear wall (collector).
4. Design boundary members and supporting elements of the structure.
5. Design wall stud bracing (reference AISI’s Wall Stud Design Standard).
6. Determine the overturning restraint (holdown) and anchorage required.
7. Analyze top of shear wall horizontal displacement (story drift) to determine compliance with code requirements and adjust as necessary. Note that one may have to verify the initial load distribution based on the final shear wall stiffness if a rigid diaphragm is used.
8. Design the foundation for all induced forces, including anchorage embedment and transfer of overturning compression.

Deflection. Consideration of top of shear wall horizontal deflection is important, whether the wall is governed by wind or seismic forces, as excessive deflection can lead to unsightly cracks or failures in finish materials such as stucco, gypsum board, and glass windows. In addition, excessive deflection can lead to member or assembly failure and collapse.

Currently, there is no code drift limit for walls loaded in-plane for wind. However, the commentary section CB.1.2 of American Society of Civil Engineer’s Minimum Design Loads for Buildings and Other Structures (ASCE 7-02) states, "An absolute limit on inter-story drift may also need to be imposed in light of evidence that damage to nonstructural partitions, cladding, and glazing may occur if the inter-story drift exceeds about 3/8 inch, unless special detailing practices are made to tolerate movement." For seismic loading, the drift limit is checked at the anticipated "real" position of the shear wall as it undergoes some sort of yielding, or inelastic response, to the design earthquake. To accomplish this, the codes require the amplification of drifts computed by the LRFD method by a factor of 0.70R for UBC designs (denoted as Cd for International Building Code (IBC) designs). This translates to a limit of approximately 1/2 inch for an 8-foot-tall wall, if using the ASD method.

The AISI standard provides a deflection equation for blocked, cold-formed steel framed, wood or steel sheathed shear wall assemblies. This equation is a function of four basic components: linear elastic cantilever bending, linear elastic sheathing shear, non-linear effects, and holdown deformation. The vertical deflection due to holdown deformation is to be multiplied by the shear wall height-to-width ratio (h/w) to obtain the hold-down contribution to the top of wall horizontal drift, as shown in Figure 3.

This deflection equation is for a Type I shear wall. However, the 2003 IBC states that one may compute deflection of wood framed shear walls with openings (Type II) by taking the maximum individual deflection of shear wall segments and dividing it by the shear resistance adjustment factor used in the design of the Type II shear wall. This same methodology appears appropriate for cold-formed steel framed shear wall assemblies as well.

Special seismic requirements

The 2003 IBC assigns an R-value of 6.5 for light-framed wood or steelsheathed shear wall assemblies with no building height limit for Seismic Design Category (SDC) A through C and a 65-foot building height limit for SDC D through F. As the R-value is high and, therefore, the seismic design load is low, the codes and standards require special design and detailing considerations to better ensure ductile behavior of the lateral system. The standard specifies special requirements when one determines the design seismic forces for a cold-formed steel shear wall using an R-value greater than 3.0.

Both the code and standard requirements include that the strength of connections (top chord splices, boundary members, and collectors), the boundary members, and the anchorage be designed for the amplified seismic loads (over-strength factor) or the maximum force that the system can deliver. As mentioned previously, this is to prevent sudden failure, such as end post buckling or a connection failure, and better ensure ductile behavior of the assembly.

The maximum force the system can deliver typically is considered to be the ultimate strength of the member delivering the load to the shear wall or the ultimate (tabulated nominal) shear wall strength.When designing light-framed shear walls in a bearing wall system using the 2003 IBC and an overstrength factor of 3.0, the over-strength factor requirement becomes less restrictive than the "maximum the system can deliver" requirement only when the design seismic load is 56 percent or less of the shear wall strength. This is considering the case when the shear wall strength is the maximum force the system can deliver. Note that the foundation need not be designed for the amplified seismic loads.

Seismic design for SDC A through C. The 2003 IBC permits one to use an R-value of 3.0 and not 6.5 for "steel systems not detailed for seismic" in SDC A through C. Therefore, the special seismic requirements do not apply and either the published wind or seismic shear wall assembly strength table values may be used as one is increasing the design load by decreasing the R-value. The designer must still multiply the shear wall table nominal values by the appropriate resistance factor or safety factor to obtain the available design strength when LRFD or ASD, respectively, is used.

The 2003 IBC permits the use of "light-framed walls with shear panels of all other materials" such as gypsum board. However, they are assigned a low R-value of 2.0 because of the brittle nature of these sheathing materials, and are restricted to a building height of 35 feet in SDC D and not permitted in SDC E through F.

Conclusion

As discussed in this article, the current codes and AISI standards and commentaries developed during the last several years provide a wealth of additional information and clarification for the designer and builder of cold-formed steel lateral-force resisting systems.

Jeff Ellis,P.E., S.E., is a branch engineer for the southwest region of Simpson Strong-Tie Co., Inc. He currently serves on the LGSEA Board of Directors and is a member of the Structural Engineers Association of California (SEAOC), the AISI Committee of Framing Standards (COFS), and the AISI COFS Lateral Design Task Group. He can be reached at jellis@strongtie.com.